In recent years, a method of directly electrolyzing metal chloride to obtain metal and chlorine has heretofore been proposed. Unlike a production method with electrolysis conducted using an aqueous solution containing metal chloride, such a production method has characteristics of obtaining chlorine with high purity as high as 100% and metal with high purity. Thus, such a production method can be used not only for applications to the production of metal but also for collecting reduction metal used when metal is obtained from metal chloride.
More particularly, as examples of metals obtained from metal chloride, alkali metals, such as sodium, and aluminum have heretofore been known. As examples of reduction metals collected upon reducing metal chloride, further, metal such as magnesium or the like has been known for use in refining titanium from titanium chloride upon using a so-called Kroll's method.
Further, a method of obtaining high purity silicon upon reducing silicon tetrachloride with zinc using a so-called zinc reducing method enables a compact equipment to be used with low energy consumption for obtaining high purity silicon with six nines or more and hence, has been getting a lot of attention as a method of producing silicon for a solar cell demand of which is expected to rapidly expand in the future.
Although such a method is implemented under a reaction indicated by Chemical Formula 1 expressed below, a molecular weight of zinc chloride (ZnCl2) is 136.4 with respect to silicon (Si) having an atomic weight of 28.1 and also, zinc chloride is produced with two molecules. This results in the production of zinc chloride with a yield about ten times that of silicon in yield, causing a serious issue to arise with the establishment of a collection processing method for zinc chloride.SiCl4+2Zn→Si+2ZnCl2  (Chemical Formula 1)
The present inventors have focused attention on zinc chloride already having a melting point ranging from 283° C. to 360° C. and zinc having a melting point of 413° C. and the like, and resultantly, found a condition under which molten zinc chloride can be directly electrolyzed. More particularly, it has been found that although the melting point of zinc is higher than the melting point of zinc chloride by 100° C. or more, molten zinc chloride can be directly electrolyzed at high efficiency in a temperature range varying from 500° C. to 550° C. higher than the melting point of zinc chloride by about 200° C. or more on consideration of electric conductivity and viscosity coefficient of zinc chloride electrolyte. However, a vapor pressure of zinc chloride increases up to a level of about 0.05 atm and chlorine gas is generated accompanied by the occurrence of a large amount of mist, resulting in a tendency of causing a phenomenon to occur with the clogging of delivery pipes unless otherwise addressed.
To address such an issue, Patent Document 1, indicated below, proposes an electrolysis apparatus including electrolysis electrodes of a bipolar type to have increased electrolysis efficiency, and a demister located on an electrolysis vessel at an upper portion thereof and having the nearly same cross-sectional area as that of the electrolysis vessel for thereby lowering an uplift speed of chlorine gas containing metal mist while cooling chlorine gas during the uplift of chlorine gas to cause fine liquid droplets of zinc chloride in chlorine gas, i.e., zinc chloride mist, to drop into an electrolytic bath.
Further, Patent Document 2, indicated below, proposes an electrolysis apparatus arranged to surround electrodes with an electrode frame to keep a temperature of a surface of electrolyte at a temperature lower than an actual electrolysis temperature for thereby suppressing the occurrence of zinc chloride mist.
[Patent Document 1] Japanese Patent Application Laid-Open Publication 2005-200759
[Patent Document 2] Japanese Patent Application Laid-Open Publication 2005-200758